1) Field of the Invention
This invention relates to ball screw devices suitable for use as actuators in machine tools, scalar robots or other industrial equipment, and especially to a ball screw device small in dimensions, light in weight, excellent in performance and durability and convenient in use and also to a ball screw device capable of inhibiting lubricant scattering to the outside, thus effectively meeting the demand for a cleaner ball screw device.
2) Description of the Related Art
A conventional ball screw device of the above type is disclosed, for example, in U.S. Pat. No. 4,939,946 issued on Jul. 10, 1990. As is illustrated in FIG. 1, on a single shaft 102 provided with a ball screw groove 103 and ball spline grooves 104 formed on an outer peripheral wall thereof, a ball screw nut 105 is fitted with a number of balls 126 interposed therebetween and a ball spline nut 106 is also fitted by way of a number of balls 130 interposed therebetween. Both the nuts 105,106 are rotatably assembled on a common housing 112, each by means of a combination of support bearings 120,121. Pulleys 123,123 are also provided to drive the nuts 105,106 independently of each other. The support bearings 120,121 in each combination are constructed as angular contact bearings and are arranged in a back-to-back relationship. Interposed between the respective bearings 120,121 are spacers 131 so that a so-called tensile pre-load is applied to balls of the support bearings 120,121. The pre-load is adjustable by choosing spacers 131 of appropriate axial width. This adjustment of the pre-load is said to prevent the development of rattling due to elastic deformation of the balls even when the shaft 102 is suddenly stopped during reciprocation. The interposition of the spacers 131, however, has led to an increase in the number of parts. Moreover, it is impossible to reduce the radial thickness of the spacers 131 so that the ball screw device unavoidably has a greater outer diameter. In addition, the nuts 105,106 have a longer axial dimension because of the inclusion of the spacers 131. As a consequence, the ball screw device cannot avoid an appreciable increase in overall external dimensions.
As described above, the support bearings 120,121 which in combination support the ball screw nut 105 or the ball spline nut 106 are constructed as angular contact bearings. The support bearings 120,121 in each combination are arranged in the back-to-back relationship (DB arrangement) with the spacer 131 interposed therebetween, whereby a so-called tensile pre-load is applied. The construction of the conventional ball screw device described above is, therefore, accompanied by the problem that it does not provide any freedom in choosing the function of the ball screw device, in other words, it does not allow the support bearings to be arranged in a face-to-face relationship (DF arrangement) to apply a compressive pre-load or to use oversized balls slightly greater than the space of the ball grooves, thus enabling each ball to provide contact with its associated ball groove at four points, to apply a so-called "P pre-load".
Further, in the conventional ball screw device described above, neither the ball screw nut 105 nor the ball spline nut 106 is provided with any particular oiling system for the lubrication of the balls but relies merely on grease being applied. Filling of excess grease may result in grease scattering to the outside so that the outside may be contaminated. On the other hand, inadequate greasing may lead to poor lubrication and may hence require frequent grease replenishment, resulting in the problem that its maintenance would be time-consuming.
For example, when a robot equipped with such a ball screw device is used in a clean room, it is of extreme importance to prevent possible contamination of the clean room by scattered lubricant. This has conventionally been dealt with by simply applying to bearings a grease of high viscosity and high adhesion. This grease, however, presents the problem that it is inferior in lubrication life and rust-prevention to general grease.
There is also the practice of continuous evacuation of a ball screw device by a vacuum means while using general grease, so that any scattering of lubricant can be prevented. In such a conventional evacuation method, the ball screw device is enclosed in a casing and the entire interior of the casing is evacuated. This, however, requires the evacuation of a large space, resulting in the problem that the evacuation is not fully effective to prevent lubricant scattering, even with provision of a large vacuum means.